Wrapped film balloons are typically made by wrapping a film tube at a large second diameter (approximate to the desired nominal diameter) and then drawing the film tube down to reduce it to a first diameter and then compressing to store length to allow for subsequent fibril reorientation upon inflation. While this process is very involved, all of the subsequent steps are in order to reduce its diameter to a point making it suitable for mounting on a catheter.
An alternate method of making a balloon would be to wrap a layered membrane construct directly at a smaller, first diameter (a diameter approximate to the delivered diameter). In order to construct a wrapped film balloon at a smaller 1st diameter, the film of which the balloon is constructed would need to distend 300-700% or more at least along the direction oriented around the circumference. A film that could distend 300-700% or more would facilitate construction of a balloon or balloon cover wrapped directly at a smaller (1st) diameter. That is, it could be circumferentially wrapped in a tube at a small first diameter so that the distensible direction of the film is oriented along the circumference of the balloon. Upon inflation, the circumference would grow, distending the film from a 1st diameter to a 2nd diameter.
Previous film wrapped balloon or balloon covers capable of being constructed at a first diameter and distensible up to a nominal diameter (300% to 700% or more) were highly anisotropic films where the weak direction was oriented circumferentially to allow for distension. Balloons of this construction have some limitations which make them less than optimal.
For example, because the strong direction of the anisotropic film provides strength to the balloon wall in the longitudinal direction, the balloon wall is limited in its ability to distend in the longitudinal direction to account for inflation. As the balloon is inflated, the longitudinal distance from seal to seal increases because of the inflated profile. As the underlying balloons forms its inflated shape, the cover needs to grow in the longitudinal direction to account for this path change. This path change is greater for larger diameter balloons and for balloons with steeper cone angles.
Compounding this issue seems to lie in the tendency for a highly anisotropic film to foreshorten in the longitudinal direction to allow for radial distention. So while the balloon wall tends to lengthen to account for the path length change, the balloon wall tends to foreshorthen to allow for radial distension because of the anisotropic film. The degree of foreshortening increases for increasing balloon diameters. This inability to distend longitudinally and the tendency to foreshorten impacts the ability to inflate fully and causes unwanted stress on the material some of which is transferred to the catheter causing buckling and in the case of a cover, to the underlying balloon causing cone rounding.
Some remedial measures can be made in attempts to minimize the undesired effects of these tendencies such as manually storing length by longitudinally compressing the material. However, this is an extra processing step, and adds bulk to the cover. Also, this extra length that can be stored is relatively mobile and can migrate/bunch undesirably during processes such as sheath insertion.
Another limitation of the above described constructs relate to the weak direction being oriented circumferentially. Such balloon or balloon covers have a very limited ability to influence the final burst properties of the balloon as the material will continue to distend until it splits with very little force.
Therefore, a circumferentially wrapped balloon or balloon cover, wrapped at a small (1st) diameter, that can reduce the amount of foreshortening, can allow for an appropriate amount of longitudinal lengthening during high amounts of a radial distention (to allow for full inflation) can be beneficial, and can provide increased wall strength along the circumferential direction to increase burst.